1. Field of the Invention
The present invention relates to an EGR system for an internal combustion engine, and more specifically, to a technology of diagnosing a malfunction of an EGR apparatus.
2. Description of the Related Art
A well-known EGR apparatus (exhaust gas recirculation system) prevents the generation of NOx (nitrogen oxide) by circulating a portion of exhaust gas, which is discharged from an internal combustion engine, through an exhaust path to an intake path as EGR gas (exhaust gas recirculation gas), and then reducing the combustion temperature.
The EGR apparatus is generally formed of an EGR passage connecting the exhaust path to the intake path and an EGR valve for controlling the passage sectional area of the EGR passage. The EGR apparatus controls the flow rate of EGR gas by adjusting the opening angle of the EGR valve according to the operation state of the internal combustion engine.
Because the EGR gas is a high-temperature gas that contains various exhaust gas components, the EGR valve is placed in a high temperature environment and might get broken or fail to operate properly. If the EGR valve has a malfunction or other troubles, the flow rate of the EGR gas cannot be properly controlled. This makes it impossible to prevent the generation of NOx, and moreover could incur abnormal combustion of the internal combustion engine.
For this reason, various technologies of diagnosing a malfunction of the EGR apparatus have been developed. For instance, there is a well-known system having a structure in which, when placed in an internal combustion engine that is installed in a vehicle and capable of activating a fuel cut during vehicle deceleration, the system completely closes a throttle valve while opening an EGR valve during fuel cut, and decides that the EGR apparatus has a malfunction if the intake manifold pressure hardly changes before and after the opening operation of the EGR valve (Unexamined Japanese Patent Publication No. 2-75748).
The above structure reduces an impact on the output of the engine and makes it possible to diagnose a malfunction of the EGR apparatus without deteriorating drivability.
Engine speed is decreased during the vehicle deceleration, whereas the intake manifold pressure is increased in proportion to the decrease of the engine speed. The technology described in the above publication has not been developed in consideration of the amount of a spontaneous change in the intake manifold pressure, and therefore is not capable of performing a malfunction diagnosis with accuracy.
The complete closing of the throttle valve during fuel cut raises the intake manifold pressure and therefore the cylinder pressure in the intake stroke of the engine toward a negative pressure side. This incurs an oil leak into the combustion chamber from a gap between a piston and a cylinder, thereby increasing oil consumption and adversely affecting gas emissions. In light of these problems, another system has lately been developed, which implements negative pressure reducing control during fuel cut. To be concrete, in a high engine speed region where cylinder negative pressure is increased, an electric throttle valve is used so that the throttle valve may be controlled to be opened at a minute opening angle rather than be totally closed, or an idle speed controller is controlled to a slightly open side. By so doing, the intake manifold pressure is controlled to an ambient pressure side. Consequently, oil is prevented from leaking into the combustion chamber.
If the negative pressure reducing control is carried out as described above, the throttle valve and the idle speed controller are operated, and therefore, the intake manifold pressure is different from the case where the negative pressure reducing control is not performed. Even if the amount of the spontaneous change of the intake manifold pressure is taken into account, it is impossible to directly apply the amount of the spontaneous change that is caused when the negative pressure reducing control is not performed. For this reason, the technology described in the publication does not achieve an accurate diagnosis for a malfunction of the EGR apparatus.
In this case, for example, it is possible not to diagnose a malfunction of the EGR apparatus as to the high engine speed region where the negative pressure reducing control is performed. However, this limits the malfunction diagnosis to be performed only when the engine speed is low, which is undesirable because the diagnosis is drastically reduced in frequency, and the requirements of the OBD (On Board Diagnosis) system are not then fully satisfied.